It is, of course, generally known to utilize a multilayer film structure having a plurality of desirable properties. For example, one of the layers in the multilayer film structure may be a barrier layer that prevents or otherwise restricts molecules of a gas, a vapor or a flavor of a food article from diffusing therethrough. The barrier layer may be a “core” of the multilayer film structure and may be surrounded by a number of other film layers, each of the layers having a plurality of characteristics. For example, an abuse layer may be provided on an outside of the film structure for adding a property that causes the film to resist tearing, scratching and/or cracking. Additionally, a sealant layer may be provided on an alternate surface of the film structure for providing a layer that may seal to itself or to other layers or articles upon heating. Further, multiple layers may be contained within the film structure having a plurality of “tie” or adhesive layers for bonding the internal layers, such as the barrier layer, to the abuse layer, the sealant layer or any other layer within the multilayer film structure.
A typical barrier layer may include a copolymer of polyvinylidene chloride (“PVdC”) and an alkyl acrylate, such as, for example, methyl acrylate (“MA”). This flexible film formulation is typically referred to as MA-Saran and is produced by the Dow Chemical Company (“Dow”). The addition of methyl acrylate to PVdC provides improved thermal stability to the PVdC, as PVdC itself is relatively unstable at temperatures required for producing a film via, for example, blown-film or extrusion lamination. General information on PVdC, MA-Saran and other related films including properties thereof can be found in “Vinylidene Chloride and Poly(vinylidene Chloride)” Kirk-Othmer: Encyclopedia of Chemical Technology, 3rd Ed., Vol. 23 (New York: John Wiley & Sons, Inc. 1983), pp.764-798.
Although PVdC is relatively useful due to its low permeability to gases and vapors such as oxygen and water vapor, its thermal instability results in degradation at temperatures at or around the melt temperatures of the PVdC-MA. Further, irradiation of the film layers for cross-linking of polymer chains within adjacent layers of the multilayer films may cause PVdC to degrade as well. Light, such as UV radiation, may also cause the PVdC to degrade. The degradation reaction may produce HCl as a by-product along with the formation of a conjugated polyene. While the addition of the alkyl acrylate reduces the degradation process, heat and/or radiation still may cause significant degradation.
The degradation reaction generally proceeds as follows:(CH2CCl2)n→(CH═CCl)n+nHCl
In addition to the production of hazardous by-products such as HCl, the degradation may also cause a decrease in the crystallinity of PVdC polymer or copolymer structure, thereby increasing the potential for gas or vapor transmission therethrough. Therefore, the heat necessary to extrude PVdC or bond other layers to the barrier layer, as well as radiation utilized to cause cross-linking, may lower the quality of PVdC as a barrier material.
In addition, the formation of conjugated polyenes causes a film produced by PVdC to discolor from clear to yellow. If enough degradation occurs, PVdC film may turn brown or black. Specifically, optical properties of the film are greatly reduced due to the degradation of PVdC by heat, light or electron beam irradiation.
As noted in the Kirk-Othmer article, stabilizing PVdC is fairly developed. Specifically, dienophiles such as, for example, maleic anhydride and dibasic lead maleate, have been found to prevent discoloration of PVdC films by reacting with and stabilizing the color-producing conjugated dienes within the polymer. Dienophiles generally stabilize these conjugated polyenes by reacting with the double bonds in multiple Diels-Alder reactions. The reactions remove the conjugated double bonds, thereby improving the properties of the film, especially optical clarity. A further advantage of using a dienophile is that HCl remains within the film thereby slowing the progress of the reaction.
However, formulations of PVdC with dienophiles besides the ones mentioned above have rarely been utilized up to this point. U.S. Pat. No. 5,679,465 to Bekele teaches using a dienophile that is a copolymer having an anhydride moiety. Specifically, Bekele discloses a dienophilic terpolymer having olefinic, acrylic, and anhydride comonomers, or a grafted copolymer of maleic anhydride. Further, a preferred embodiment of Bekele includes an ethylene/alkyl acrylate/maleic anhydride terpolymer, such as ethylene/butyl acrylate/maleic anhydride terpolymer. However, dienophilic copolymers are difficult to use and may interfere with the crystallinity of the PVdC film matrix, hereby decreasing the barrier properties of the film. Further, cheaper and more effective dienophiles, along with processes of combining the dienophiles with the PVdC formulation are necessary.
Therefore, a need exists for an improved PVdC formulation having dienophiles incorporated therein for flexible film packaging that will react with conjugated polyenes formed by the degradation of PVdC by heat, light and electron beam irradiation. Further an improved flexible film package and method of manufacturing the same is needed.